Influence of Al AddItIon on the thermAl

stAbIlIty And mechAnIcAl propertIes of

fe76.5-xcu1si13.5b9Alx Amorphous Alloys

y.y. sun, m. song #

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

(Received 27 July 2011; accepted 31 August 2011)

Abstract

This paper fabricated Fe76.5-xCu1Si13.5B9Alx (x=0,1,2,3,5,7 at.%) amorphous ribbons using single-

roller melt-spinning method. The effect of Al content on the thermal stability and mechanical properties was

investigated. The results indicated that Al addition have little effect on the amorphous formation ability of the

alloys. On the other hand, increasing the Al content can substantially increase Tx2, which corresponds to the

crystallization of Fe borides. Nanoindentation tests indicated that hardness of the alloys increase slightly with

increasing the Al content, and Young’s modulus has a complicated relationship with the Al content.

Keywords: Fe-Cu-Si-B alloy; Nanoindentation; Amorphous; Shear bands

# Corresponding author: min.song.th05@alum.dartmouth.org

J o u r n a l o f

M i n i n g a n d

M e t a l l u r g y

J. Min. Metall. Sect. B-Metall. 48 (1) B (2012) 45 - 51

1. Introduction

Fe is one of the most wildly usedcomponents in magnetic materials. Due tothe rapid development in some fields, such aselectronic communication, automation andrecording media, extensive attentions havebeen paid to Fe-based alloys [1,2]. Thefamily of Fe-Cu-Nb-Si-B alloys (also called

Finemet alloys) is a kind of excellent softmagnetic materials and have widely beenused in industrial applications [3,4]. Thistype of the alloys is typically produced fromamorphous precursors that are partiallydevitrified, resulting in nano-scaledferromagnetic grains embedded in theresidual amorphous matrix. However, theprice of producing Finemet alloys is very

DOI:10.2298/JMMB110727009S

high due to the high cost of the Nb element.Thus, the Nb-free soft magnetic alloys havebeen developed, such as Fe-Cu-B, Fe-Cu-Si-B (Ohta, 2007) [5-7] nanocrystalline softmagnetic alloys with improved saturationmagnetization (Ms) as high as 1.8 T, for theeconomical and practical considerations. Atthe same time, substitution of Nb by othercheap elements is another effective way tocut the cost. Previous works showed that thesubstitution of Nb by Al element, with thesimilar atom radius (0.143 nm) to Nb, inFinemet alloys has a great effect on themagnetic properties, such as increasing thesaturation magneticzation (Ms) [8-10],decreasing the coercivity (Hc) [9-13],enhancing the Curie temperature when the Alcontent is up to 3 at.% [14] and decreasingthe magnetocrystalline anisotropy K1 [9,15].However, almost all the investigationsconcentrated on the magnetic properties ofthe Fe-Cu-Si-B-Al alloys, and not muchinformation exists on the effects of the Aladdition on the structures and thecorresponding mechanical properties.Therefore, in this work, the effects of Aladdition on the mechanical properties of Nb-free Fe-Cu-Si-B amorphous alloys werestudied using X-ray diffractometer (XRD),transmission electron microscope (TEM),differential scanning calorimetry (DSC) andnanoindentation technique.

2. experimental

Master alloy ingots with the nominalcomposition of Fe76.5-xCu1Si13.5B9Alx (x=0,1, 2, 3, 5, 7, atomic percentage) wereprepared by arc melting high-purity Fe(99.9%), B (99.9%), Si (99.9%) , Al (99.9%)

and Cu (99.9%) in a Ti-gettered high-purityargon atmosphere. The ingots were re-meltedfive times and stirred by a magnetic beater toensure its compositional homogeneity. Melt-spun ribbons with 20 μm in thickness and 1.5mm in width were prepared by single-rollermelt-spinning method under an argonatmosphere with a speed of about 50 m/s.The fabricated ribbons were very flexibleand could be bent by 180˚ without cracking.

The microstructures of the ribbons werestudied using a Dmax 2500VB X-raydiffractometer (XRD) with Cu-Κα radiationand a JEOL 3000F field emissiontransmission electron microscope (TEM).Crystallization kinetics of the alloys wasinvestigated by differential scanningcalorimetry (DSC, NETZSCH STA 449C)with a scanning rate of 0.33 K/s. Mechanicalproperties were studied using an UltraNanoindentation tester (CSM) with themaximum loads as high as 10 mN. Theloading and unloading procedures werecarried out under a load control of 20 mN perminute, with the dwell period of 15 secondsbeing imposed before unloading. Berkovichtip was impressed into the longitudinal crosssection of the ribbons, which weremechanically polished with 1/4 μm and 1 μmdiamond abrasives firstly. At least 10 testswere performed on each sample to obtainenough statistical significance on thehardness and Young’s modulus.

3. results and discussion

3.1. X-ray diffraction and microstructures

Figure 1 shows the XRD patterns of thefabricated Fe76.5-xCu1Si13.5B9Alx ribbons

Y.Y. Sun / JMM 48 (1) B (2012) 45 - 5146

with x= 0~7 at.%. The patterns consist ofbroad halos without obvious detectablepeaks, indicating the presence of a singleamorphous phase within the detection limitof XRD.

Figure 2(a) and the inset show a bright-field image and the corresponding selectedarea diffraction (SAED) pattern of theFe75.5Cu1Si13.5B9 Al1 (x=1) ribbon. Theabsent of contrast in bright-field image and abroad diffuse halo in SAED pattern indicatethe formation of a completely amorphousstructure. The high-resolution transmissionelectron microscope (HRTEM) image inFigure 2(b) can further confirm theamorphous structure due to the disorderlattice arrangement. It should be noted thatsimilar features were also observed for therest alloy ribbons (x = 0~7 at.%). Thisindicates that all the alloys with different Alcontents have good amorphous formationability.

3.2. dsc curves

Figure 3 shows typical DSC curves of theFe76.5-xCu1Si13.5B9Alx ribbons with aheating rate of 0.33 K/s and the relationshipbetween Tx1 and Tx2 and Al content. It can beseen from Figure 3a that all the amorphousribbons did not show an obvious glasstransition before crystallization, indicatingthat the Fe76.5-xCu1Si13.5B9Alx ribbons arenot glassy but ordinary amorphous alloys[16].

Two distinct exothermic peaks on the

Y.Y. Sun / JMM 48 (1) B (2012) 45 - 51 47

Figure 2: (a) Bright-field TEM image and the

corresponding SAED patterns of the melt-spun

Fe75.5Cu1Si13.5B9 Al1 ribbon and (b) HRTEM

image.

Figure 1: XRD patterns of the Fe76.5-

xCu1Si13.5B9Alx ribbons.

curves indicate that the crystallization of allstudied ribbons is through a two-stepprocess. It should be noted that the firstcrystallization peak corresponds to theformation of α-Fe(Si) phase, and the secondone corresponds to the formation of Fe

borides, according to the previousinvestigations [17,18]. It should also benoted that the onset temperature Tx1 of thefirst crystallization process remains almostconstant (around 734 K within the detectionlimit of the DSC), while the onsettemperature Tx2 of the second crystallizationprocess increase dramatically withincreasing the Al content. This phenomenonindicates that Al addition has little effect onthe amorphous formation ability and/or theformation of crystalline α-Fe(Si) phase dueto the relatively stable Tx1 with increasingthe Al content. On the other hand, theincreased Tx2 with increasing the Al contentwas also observed in Fe-Al-B ternaryamorphous alloys, in which thecrystallization temperature corresponding tothe formation of Fe borides increases withincreasing the Al content [19]. It is wellknown that crystallization of amorphousalloys is a nucleation/growth process, whichis dominated by the diffusion of base metals.It can thus be concluded from Figure 3 thatthe addition of Al might substantially inhibitthe diffusion of B instead of Fe, since theformation temperature of single phase α-Fe(Si) remains constant, while that of the Feborides increases dramatically withincreasing the Al content.

3.3. mechanical properties

Figure 4a shows the typical load-displacement (P-h) curves of specimens withdifferent Al contents under a load of 10 mN.It can be seen from Figure 4a that the loadingparts of all the curves are relatively smoothwithout obvious pop-in events, differingfrom many other amorphous systems such as

Y.Y. Sun / JMM 48 (1) B (2012) 45 - 5148

Figure 3: (a) DSC curves of the Fe76.5-

xCu1Si13.5B9Alx ribbons with a scanning rate of

0.33K/s and (b) the relationship between Tx1 and

Tx2 and Al content.

Zr-based, Pd-based and Mg-based metallicglasses [20,21]. Wright et al. [22,23]concluded that each pop-in corresponds tothe operation of a single shear band thatquickly accommodates the applied strain inBMG, and larger pop-in displacementcorresponds to greater shear displacementswithin the shear band. A previous study [24]indicated that the absence of pop-in duringnanoindentation for Fe-Cu-Si-B amorphousalloys might be due to the small size of shearbands (less than 1 μm), and the simultaneousoperation of the multiple shear bands.

Figure 4b shows the dependence of theYoung’s modulus and hardness obtainedduring nanoindentation as a function of Alcontent. It can be seen from Figure 4b thatthe evolution of the Young’s modulus withAl content is very complicated. The Young’smodulus has the maximum value of 164.8GPa and minimum value of 122.5 GPa whenx=0 and x=2, respectively. It is widelyaccepted that Young’s modulus is one of thefundamental elastic properties representingthe interatomic distance and bondingstrength between atoms [25]. The evolutionof Young’s modulus with Al contentindicates that Al atom has a relative complexeffect on the bonding strength in Fe-Cu-Si-Balloys.

It can also be seen from Figure 4b thatincreasing the Al content can slightlyimprove the hardness from ~9 GPa to ~9.6GPa when the Al content increases from 0%to 7 at.%, except for the specimen with 2at.% Al addition, which has an abnormalhigh hardness. It is known that the plasticdeformation of amorphous alloys is throughthe nucleation and propagation of the shearbands [26]. These microscopic shear bands

generated during nanoindentation can beobserved using atom force microscope(AFM).

Figure 5 shows a typical AFM image ofthe Fe75.5Cu1Si13.5B9 Al1 ribbon afternanoindentation. It can be seen from Figure5 that semi-circle patterns around indentedhole are observed (arrowed in Figure 5) andcorrespond to the shear bands, which arenucleated outside of the indents when theindenter moves deeper into the sample, andpile up as the material flow upwards andaway from the depth of the indents. The

Y.Y. Sun / JMM 48 (1) B (2012) 45 - 51 49

Figure 4: (a) load-displacement curves of Fe-

Cu-Si-B-Al alloy and (b) the relationship

between hardness and Young’s modulus and Al

content.

semi-circle patterns were observed for all thespecimens with different Al contents. It canbe seen that the semi-circle patterns aroundeach side of indented hole are almostsymmetrical, resulting from thesimultaneous nucleation and propagation ofseveral shear bands under the sharpBerkovich indenter during nanoindentation.Wright et al. pointed out that each pop-inevent (sudden displacement excursion)occurred on load-displacement curvecorresponds to the operation of a single shearband [22,23]. Due to the simultaneousoperation of shear bands, the load-displacement curves of all studiedamorphous alloys appear relatively smooth,as shown in Figure 4a, and no obvious pop-in was detected. The similar result was alsoobserved in Fe-Cu-Si-B bulk metallic glassduring nanoindentation [24].

4. conclusion

In this work, Fe76.5-xCu1Si13.5B9Alx(x=0,1,2,3,5,7 at.%) amorphous ribbonswere prepared by single-roller melt-spinningmethod and the effect of Al content on thethermal stability and mechanical propertieswas investigated. The results indicated thatAl addition have little effect on improvingthe amorphous formation ability. However,increasing the Al content can increase Tx2,which corresponds to the crystallization ofFe borides. The hardness of Fe76.5-xCu1Si13.5B9Alx alloys increase slightly withincreasing the Al content.

Acknowledgements

This work is financially supported byHunan Provincial Natural ScienceFoundation of China (11JJ2024), theProgram for New Century Excellent Talentsin University (NCET-10-0842) andFundamental Research Funds for the CentralUniversities (2011JQ021).

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